A multiple burner arrangement is disclosed with a multiplicity of individual burners which are designed as premix burners and which serve for firing a combustion chamber for a thermal engine, preferably for a gas turbine plant, and each have a swirl space into which combustion supply air and fuel are fed so as to form a swirl flow, the swirl flow forming downstream of the premix burner, within the combustion chamber, a backflow zone which forms spatially in a largely stable manner and in which a burner flame is formed after the ignition of the fuel/air mixture. A method for operating a multiple burner arrangement of this type is likewise described.
Multiple burner arrangements have gained acceptance not least because of ecological factors, since the formation of nitrogen oxides in the exhaust gases can be kept low on account of a low flame temperature along with a high air excess. In this connection, in particular, it has become possible for annular combustion chambers, as they are known, to become established, which are employed for the purpose of driving gas turbine plants and provide a multiplicity of individual premix burners in a circular arrangement around the rotating components of a gas turbine, the hot gases of which are supplied directly to the following turbine stage via an annularly designed flow duct.
An annular combustion chamber arrangement of this type may be gathered, for example, from EP 597 138 B1, which provides a multiplicity of annularly arranged premix burners, such as may be gathered, for example, from EP 387 532 A1, these being designed in each case as double cone burners which provide a swirl space surrounded radially by two hollow conical part bodies, the respective center axes of which are arranged so as to be offset relative to one another, so that adjacent walls of two conical part bodies enclose in their longitudinal extent tangential slots for the combustion air. Via a fuel nozzle arranged largely centrally within the swirl space, liquid fuel can be fed into the axially conically widening swirl space. Likewise, the premix burner can be supplied with gaseous fuel via gas inflow ports distributed along the tangential slots within the wall of the two conical part bodies. Mixture formation with the combustion supply air thus already takes place in zones of the inlet slots, as homogeneous a fuel concentration as possible over the entire cross section of the swirl space occurring along the swirl flow propagating axially within the swirl space. This gives rise at the burner outlet to a defined backflow zone which is in the form of a spherical cap and at the tip of which ignition takes place so as to form a burner flame spatially stable within the zone.
During the operation of a gas turbine plant of this type, as a rule, during the starting of the gas turbine and in low load ranges, the fuel supply for each individual premix burner is carried out via what is known as a pilot stage which, depending on the design of the premix burner, is designed as a central burner lance, such as is described, for example, in DE 196 52 899 A1, or as a pilot gas supply provided directly at the burner outlet, upstream of the combustion chamber in the flow direction.
In both instances, fuel is administered directly into the flow zone required for flame stabilization, but, in terms of pollutant emission, burns in an extremely unfavorable mixture ratio under virtually stoichiometric conditions. On account of the NO2, CO and NOx emission values which are high in what are known as pilot operation, it is therefore necessary, particularly in the medium and upper load range of the gas turbine plant, to throttle the fuel supply via the respective pilot stage and to carry out the supply of fuel within the framework of the premix stage, as it is known, that is to say the feed of gaseous fuel along the air inlet slots through the wall of the conical part shells. After a complete shutdown of the pilot fuel supply, it is necessary to remove combustible residues from the pilot supply lines in order to avoid flame flashbacks into the pilot stage. Technically complicated scavenging methods are required for this purpose. Moreover, the changeover actions from pilot operation to premix operation, or vice versa, are undesirable, since these excite burner-internal pulsations which, depending on their markedness, subject the plant components involved in the combustion process to high mechanical load.
Furthermore, thermoacoustic oscillations of this type preferentially also arise in premix operation, that is to say in the medium and upper load range, due to which the flame stability forming within the combustion chamber is seriously impaired.
Normally, in gas turbines fired by means of annular combustion chambers, all the premix burners are supplied with gaseous fuel in the same way during premix operation. It is shown, however, that, under different load conditions of the gas turbine plant, operating ranges occur in which high combustion chamber pulsations, a poor burn-out and associated high carbon oxide values and also high values of unsaturated hydrocarbons arise and in which a poor transverse ignition behavior of the individual premix burners can be observed.
In order to counteract these problems, DE 101 08 560 A1 proposes deliberately to break up the hitherto adopted symmetry in the fuel supply of all the premix burners provided in the multiple burner arrangement, in order effectively to reduce the occurrence of combustion chamber pulsations. In this case, at least one premix burner is operated in such a way that the at least one premix burner has, within the fuel/air mixture, a spatial mixed profile deviating from all the other premix burners provided in the multiple burner arrangement. In this case, the at least one premix burner provides a fuel feed for the gaseous fuel, deviating structurally from all the other premix burners, along the conical part shells radially delimiting the conical swirl space. Although this measure contributes to the damping of pulsations in the upper load range of the gas turbine plant which are usually in resonant form and rotate circularly in an annular combustion chamber, nonetheless limits are placed on further influence on the burner behavior in terms of the operation of the gas turbine plant in different load states and taking into account other parameters influencing the combustion processes within the respective premix burners, such as, for example, highly varying moisture fractions in the combustion supply air in the case of an increase in power output of the gas turbine, ambient temperature, change in fuel composition and also aging phenomena of the overall gas turbine plant. Moreover, the proposal described above does not allow any subsequent retrofittability on already existing gas turbine plants, and therefore the known measure can be implemented solely in gas turbine plants to be newly procured.